Protective circuit for speaker windings



United States Patent [72] lnventor Lester Smith Flushing, N.Y.

[21 1 Appl. No. 673,543

[22] Filed Oct. 9,1967

[45] Patented Dec. 22, 1970 [73] Assignee Marantz Company, Inc.

Woodside, N.Y. a corporation of New York [54] PROTECTIVE CIRCUIT FOR SPEAKER WINDINGS 4 Claims, 2 Drawing Figs.

179/1 H04r 3/00 [50] Field ofSearch 179/1, 78A,

184,1A,1Mon;330/15,18,192,198,143; 317/1485, 157, 155.5, 156; 335/18 [56] References Cited UNITED STATES PATENTS 2,237,448 4/1941 Reinhard 179/1 2,959,640 11/1960 Schultz 179/1 3,142,724 7/1964 .luvonen 179/1 3,159,768 12/1964 Flanagan.... 317/1555 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Jon Bradford Leaheey Attorneys-Robert S. Dunham, R. J. Dearbom, P. E.

Henninger, Lester W. Clark, John A. Harvey, Thomas F. Moran, Gerald W. Griffin, Howard J. Churchill and John F. Ohlandt PROTECTIVE CIRCUIT FOR SPEAKER WINDINGS This invention relates to a circuit for protecting a loudspeaker and, more particularly, to a protective circuit that will prevent damage to a loudspeaker from overload currents resulting from a unique set of conditions.

Certain forms of overload protection for loudspeakers have been heretofore known in the art. For example, there has been described in U.S. Pat. No. 2,237,448 to Reinhard, a protection scheme that will obviate the danger caused by a rise in the critical temperature of the moving coil in a dynamic loudspeaker. In other words, the Reinhard system is designed simply to prevent damage produced by unforeseen temperature rises stemming from excess signal currents flowing for protracted periods. Reinhard provides that a slow acting, temperature-responsive element, of one form or another, operates to limit the current flowing to a loudspeaker coil.

A particularly vexing problem that arises in the driving of loudspeakers occurs in the situation where the final amplifier stage, i.e. the stage just preceding the speaker, is directly and conductively connected to the speaker. This contrasts with the more common situation where the output arrangement involves transformer coupling. Such direct connection, however, is a well-known useful and economic expedient for the purpose of providing a very simple output arrangement in, for example, a radio or television receiver.

r In such a direct-connected output circuit, the failure of the amplifying device, whether it is a transistor, a vacuum tube or other type of amplifying device, results in the power supply voltage being applied to the, speaker winding. The power supply conventionally takes the formof batteries or other DC supply. Since the speaker winding has a low impedance to direct current it will be damagedby the resultant excessive current unless it is protected. The failure referred to above of the amplifying device would be, for instance, the case where the device shorts out." In that event, the normally high im 'pedance afforded by the amplifying device, such as the high impedance normally present at the output of a transistor, for example, would be eliminated and thereby the speaker would receive substantially the full power supply voltage.

Accordingly, it is a fundamental object of the present invention to eliminate the danger to a loudspeaker which is posed by the fact that the speaker is directly and conductively connected to the final stage of an amplifier, thereby making the speaker susceptible to having the full value of direct voltage imposed across its windings.

. In a context or environment where the final stage of the amplifier is in the form of a push-pull circuit, as is common arrangement today, the protective device must protect against overload currents of either polarity. One example of a pushpull circuit will be set forth hereinafter, this particular form being a complementary push-pull transistor circuit, but it will be evident that other push-pull circuits, as well as single device output stages, are similarly amenable to control by the circuit of the present invention.

It is, therefore, another object of the present invention to protect a loudspeaker from currents of either polarity that may post a threat to the safety of the loudspeaker device.

In fulfillment of the above-stated objects the present invention provides as its principal feature a circuit that substantially instantaneously responds when a condition arises such that the direct voltage supply would be applied to the loudspeaker due to failure of the amplifying output stage. However, the circuit is such that it does not respond to the normal alternating signal currents that are meant to flow to the loudspeaker.

It will thus become apparent that the present invention provides a solution to a unique problem present with direct-connected loudspeakers. The circuits operation is for all practical purposes instantaneous in preventing harm that might result from the application of the full voltage supply to the speaker windings. It thus sharply contrasts with slow acting protective devices previously known in the art.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more paras illustrated in the accompanying drawing.

FIG. 1 is a circuit diagram illustrating one embodiment of invention incorporated in a typical "f the circuit of the present push-pull circuit.

FIG. 2 is an alternative embodiment of the protective circuit in accordance with the present inve'ntion.

Referring now to FIG. 1, "there is illustrated a typical transistor push-pull circuit in which the unique protective arrangement of the present invention i's jembodied. In this instance the circuit has been selected' toshow a'complementary (NPN and PNP) form of push-pull operation. In this form of circuit the transistor 10 is a PNP transistor and transistor 12 is an NPN transistor. This configuration, that is, complementary push-pull, is, of course, well-known in the art. Moreover, the direct conductive connection to the speaker at the output of this push-pull configuration is also, per se, well-known. Thus the transistors 10 and 12, which are opposite polarity transistors, have their emitters connectedto their"respective oppositely polarized power supplies, shown as batteries 14 and 16. Thence, the transistors 10 and 12 are connected in common to the load; that is, to the speaker 18 which serves as the only load in the output circuit.

The novel circuit means in accordance with the present invention is shown within the box designated 20. The elements within the box 20 are thus interposed between the final amplifying stage and the loudspeaker 18.

The protective means" generally designated 20, consists of a relay which, as shown has its coil S1 connected in series with a resistor R1. Together, these two elements are connected across the output terminals. Although the coil S1 is shown schematically, the relay is of the nonpolarized variety, that is to say, it will respond equally well to. currents of opposite polarity to perform its intended function. I 1

A capacitor C1 is connected in parallel with the relay wind ing S1. This capacitor C1 is a nonpolarized type.'The relay in the protective circuit has a contact K1 which is connected in series with the speaker 18. This contact K1 is normally closed but operates in realizing the intended objective of the protec tive circuit, so as to open the output connection the loud-.

speaker 18. An arc-suppressing capacitor C2 is connected in shunt with the relay contact terminals. I

The normal operation of the circuit of FIG. 1 is such that alternating output signals, in the form of audio frequency signals, which result from theapplication of input signals to the bases of transistors 10 and 12, are applied to the load, i.e. the loudspeaker 18, which is connectedin common to these push-pull transistor stages. The resulting alternating output currents are bypassed around the relay winding 81 through the capacitor C1. This is done in order that there be no spurious operation, that is, energization of the relay with consequent opening up of the speaker circuit when, in fact, no danger is present. 7

It should be noted that the capacitor C1 and the resistance R1 are judiciously selected so as to present sufficient impedance to alternating signals of the lowest frequency and the maximum amplitude which are intended to be supplied to the speaker. Thus, under the normal conditions, there will be no interference with the listening enjoyment.

Now, in contrast with the normal operation, let it be assumed that one of the transistors 10 or 12 fails, that is to say, one of these transistors shorts out, thus eliminating the normally high impedance which such transistor offers. In such event, considering for the moment transistor 12, substantially the full voltage of the power supply 16 would ordinarily be applied across the winding of the speaker 18. However, with the protective circuit 20 connected as shown in FIG. 1, direct current is now able to flow through the relay coil 51-. In other words, such direct current is not bypassed by the capacitor C1 but goes through the relay coil S1 energizing it and causing opening of the relay contacts K1, thereby opening the circuit to the speaker. It will be appreciated, of course, that the circuit to the speaker remains open as long as the aforenoted short circuit condition for the transistor 12 persists.

The resistance of R], that is, of the resistor in series with the relay coil 51, depends on the relay sensitivity, and should be made substantially equal to the resistance of the relay coil itself. It will be understood that the resistance of R1 is effectively acting as part of an RC circuit which determines the lowest frequency passed to the speaker and also as a current limiter for the relay.

Referring now to FIG. 2, there is illustrated an alternative embodiment of the protective circuit of the present invention. In this embodiment electronic solid-state components rather than electromechanical components, such as a relay, are employed. In particular, a device sometimes called a triac or thyristor, well known to those versed in the art, is connected to serve as the protective element which absorbs the current that would normally damage the speaker winding. The characteristics of this type of three terminal device may be appreciated by reference to US. Pat. No. 3,310,687 to Howell.

As in the first embodiment of FIG. 1, the resistor R1 and a nonpolarized capacitor C1 are again connected across the output terminals of the amplifier. However, a different control scheme or apparatus is herewith provided. The control device 30 is shown connected to the protective device 32. One terminal of the device 30 is connected to the gate 32a of the device 32 and the other end of the device 30 is connected to the common junction of the RC circuit formed by the resistor R1 and the capacitor C1. The device 32 has its output elec trodes 32b and 32c connected across the speaker winding.

The device 30, which is a two terminal device, may take the form of a so-called four layer diode and, more particularly, such a diode having the property that its VI characteristic exhibits breakdown in both the forward and reverse bias states. Effectively, then, the impedance of such a device goes to substantially zero when a predetermined critical voltage is exceeded in either direction. An alternative form for such device 30 is that it may be constituted simply of two diodes, of the type shown in U.S. Pat. No. 2,789,254, connected in series and oppositely poled.

As with the embodiment of FIG. 1, as has already been indicated, the usual alternating currents that are normally present are bypassed by the capacitor C1, so as to have no effect on the operation of device 30. However, when the DC voltage supply tends to be applied to the speaker winding, as a result of the shorting out of one of the transistors in the output stage, the device 30 immediately breaks down upon reaching its critical voltage. This critical voltage is, of course, chosen to be less than the supply voltage. The critical or breakdown voltage for device 30 is chosen to be of the order of 8 volts, while the supply voltage has a value, for example, of 36 volts. At breakdown the impedance of device 30 goes to a value of substantially zero. As a result, a biasing current flows through the gate 32a of the device 32 of a sufficient value to cause that device to assume an extremely low impedance output state, being effectively a short circuit. This means that the potentially damaging current from the DC supply is bypassed through the output of device 32, rather than flowing through the windings of speaker 18.

It is intended in this alternative embodiment of FIG. 2 that a fuse be incorporated in the output circuit so that the aforenoted short" across the device 32 will result in blowing the fuse. However, the fuse is not absolutely necessary and may be omitted since the devices 30 and 32 and the resistor R1 are capable of withstanding the current values involved.

It will be appreciated that the alternative embodiment of FIG. 2 is capable of protecting the speaker windings regardless of the polarity of DC voltage that is sought to be applied to these windings. This is so because both of the devices 30 and 32 are able to respond to absorb the potentially damaging current regardless of the applied polarity. It will be understood, of course, that the nature of the device 32 must be such that, in the illustration of FIG. 2, the polarity of current must be similar to that of the direction of conduction through the output terminals 32b and 32c. Thus, the device 32 is selected to have those modes of operation such that, for example, when a positive battery potential is sought to be applied to the point above resistor R1, the current flow will be into the gate 320 of the device 32, and this will comport with the current flow through device 32 in a direction from electrode 32b to electrode 320. As explained above, breakdown to the low impedance state at these output electrodes is accomplished through the medium of the gate control current rather than being provided directly by the DC voltage applied to the output electrodes of device 32.

In order to provide one skilled in the art with the detailed set of specifications for facilitating construction of the protective circuit of the present invention the following values of components are given:

(Fig. 1):

+B=36 v. B=36 v.

O1=200 mfd. 25 v. N.P.

C2=0.22 mfd. 250 v.

S1400 ohms D.C. relay Sigma 65Fi or equal.

(Fig. 2):

R1=220k ohms w. C1=0A=7 mfd.250 v. #30 Unilateral switch G.E. type 2N3986 or e ual. #32=Bi-directional thyristor Motorola type ac 2-4 or equal.

What has been provided by the present invention is an ex tremely simple and effective protective means which operates quickly and efficiently to protect a loudspeaker from the dangers presented by the application of the full direct voltage of the power supply. This is accomplished in such a way that the dynamic range and fidelity of the loudspeaker is not impaired and thus the listening enjoyment is in nowise diminished.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

Iclaim:

1. In a circuit where a final amplifier stage is directly and conductively connected to a loudspeaker and wherein the failure of an amplifier device in said stage causes the imposition of a direct voltage supply across said loudspeaker, the improvement comprising a protective circuit interposed between said final amplifier stage and the loudspeaker, said protective circuit comprising a controlling means which is unresponsive to the normal alternating currents flowing to said loudspeaker but which responds in the event of failure of said amplifier device to the consequent imposition of said direct voltage supply by preventing excessive current flow to said speaker due to such imposition; said protective circuit further comprising a resistor and a capacitor in series, both being connected across the output terminals of the final amplifier stage; and further comprising first and second devices, the first device having the property that its voltage-current characteristic exhibits breakdown in both the forward and reverse bias states, said second device including a gate, one end of said first device being connected to the common junction between said resistor and said capacitor, the other end thereof being connected to said gate of said second device, said second device having its output terminals across said speaker and being operative responsive to the breakdown of said first device to provide a low impedance across said output terminals thereby to absorb current that would present a danger to said loudspeaker.

2. In a circuit as defined in claim 1, the first device comprising a four layer diode and the second device comprising a thyristor.

3. A circuit comprising a loudspeaker and an amplifier stage directly and conductively connected-thereto, said loudspeaker being the only load for said amplifier stage; a direct current power supply connected to said amplifier such that, upon failure of said amplifier, said direct current power supply has its full value applied to said speaker, said circuit further comprising protective means instantaneously operating in response to such failure to prevent excessive current flow to said speaker, said means being unresponsive otherwise so as not to interfere with the flow of signal currents to said speaker; said protective means comprising a resistor and a capacitor in series, both being connected across the output terminals of said amplifier stage; and further comprisingfirst and second devices, the first device having the property that its voltagecurrent characteristic exhibits breakdown in both the forward and reverse bias states, said second device including a gate, one end of said first device being connected to the common junction between said resistor and said capacitor, the other end thereof being connected to said gate of said second device, said second device having its output terminals across said speaker and being operative responsive to the breakdown of said first device to provide a low impedance across said output terminals thereby to absorb current that would present a danger to said loudspeaker.

4. A circuit as defined in claim 3, the first device comprising a four layer diode and the second device comprising a thyristor. 

